A method includes depositing a dielectric layer over a substrate, forming carbon nanotubes on the dielectric layer, forming a dummy gate stack on the carbon nanotubes, forming gate spacers on opposing sides of the dummy gate stack, and removing the dummy gate stack to form a trench between the gate spacers. The carbon nanotubes are exposed to the trench. The method further includes etching a portion of the dielectric layer underlying the carbon nanotubes, with the carbon nanotubes being suspended, forming a replacement gate dielectric surrounding the carbon nanotubes, and forming a gate electrode surrounding the replacement gate dielectric.